1. Field of the Disclosure
The disclosure relates generally to communications and, more particularly, to communications using MEMS (microelectromechanical systems) resonators, or microresonators.
2. Brief Description of Related Technology
Modulation of a communications signal can be accomplished using many techniques, such as amplitude modulation (AM), frequency modulation (FM), pulse-width modulation, and frequency-shift-keying. For many years, broadcast radio has been based on AM and FM modulation schemes. More recently, wireless communications, such as cellular telephony, have given rise to use of other modulation techniques.
The 433 MHz communications band is a general purpose, low-power, wireless communication band. Applications in this band include automotive remote keyless entry, wireless tire pressure monitoring, remote control applications, wireless fire alarm systems, garage door openers, remote gate entry, remote window shutters, RFID, and numerous other short range, low power, wireless control and communication applications.
Two conventional communication techniques for wireless data transfer are Amplitude-Shift-Keying (ASK) modulation and Frequency-Shift-Keying (FSK) modulation. FSK modulation provides improved noise and interference immunity relative to ASK, and is the modulation technique of choice for many applications. For example, FSK modulation is used in both Europe and Japan in tire pressure monitoring applications. ASK modulation presently dominates tire pressure monitoring in the USA, although FSK modulation has been increasingly used due to better performance in interference and polarization robustness.
FIG. 1 depicts a conventional FSK transmitter circuit. In many cases, FSK modulation is implemented with a transmitter having a quartz crystal oscillator resonating at 6.78 MHz or 13.56 MHz, an oscillator circuit, a voltage-controlled oscillator (VCO) operating at 433.92 MHz, and a phase-lock loop (PLL) with either a divide-by-32 or divide-by-64 circuit depending on the quartz crystal used.
The frequency at which the quartz crystal oscillates is affected by a change in the oscillator's capacitive load. As depicted in FIG. 1, FSK modulation of the output signal is produced by switching one of two load capacitors in and out of the circuit in response to a digital data signal input. Due to the high-Q nature of the quartz crystal oscillator, however, the change in frequency (i.e., frequency pulling) that can be induced is limited. As a result, the achievable frequency pulling is typically in the range of 50-80 ppm. The frequency deviation is thus quite small, thereby limiting the amount, or depth, of modulation present in the output signal.
The sensitivity requirement for a device receiving the output signal is a function of the modulation depth achieved in the FSK link. In this way, low modulation depth FSK links lead to higher system costs.
Further costs arise from the need for temperature compensation. Without compensation, crystal oscillators would have a limited operating temperature range, as crystal temperature coefficients generally rise dramatically as temperature exceeds 85° C. Crystal oscillators are thus typically temperature-compensated, leading to increased system cost. Integrated circuits for temperature compensation of crystal oscillators are commercially available from vendors such as Infineon, Molexis, etc.
Transmitters implementing FSK modulation have also used a surface-acoustic-wave (SAW) resonator that resonates at directly 433 MHz. The frequency of the SAW-stabilized oscillator is pulled, in similar fashion as described for the crystal oscillator, using either a varactor or switched capacitors.
Unfortunately, SAW resonators exhibit frequency inaccuracy as well as a large temperature coefficient. These inaccuracies in the output frequency range lead to stricter requirements for the receiver, such as a larger intermediate frequency (IF) bandwidth. The need for a larger IF bandwidth negatively impacts receiver sensitivity, and ultimately limits the link margin for a given transmitted power.